In the fields of radio communication, digital broadcasting and the like, various multiplexing techniques are used in order to effectively utilize a limited transmission band. As one of the multiplexing techniques, a system which is called an Orthogonal Frequency Division Multiplexing (OFDM) system is proposed. In the OFDM system, many orthogonal sub-carriers are provided in a transmission frequency band and pieces of data are allocated to the amplitude and the phase of each sub-carrier to perform digital modulation.
In the case that an OFDM signal, that is, a frequency multiplexing signal which has been sent from a base station is radio-received, the OFDM signal which has been sent from the base station strikes an obstacle such as a building or a topological obstacle and is reflected and diffracted. A receiving device receives OFDM signals which have been sent passing through a plurality of paths generated by signal reflection and diffraction. The plurality of paths generated by signal reflection and diffraction are generally called a multipath. Distances over which the respective OFDM signals are propagated through the multipath are different from one another. Owing to a difference in propagated distance among the signals, the receiving device receives the plurality of OFDM signals which are different from one another in amplitude and phase.
In the case that an OFDM signal is influenced by the multipath, the amplitudes and phases of sub-carriers in the OFDM signal are changed. In order to eliminate the influence of the multipath on each sub-carrier, the receiving device performs a path estimating process to estimate the characteristic of a propagation path through which a received signal is propagated. In performing the path estimating process, the receiving device uses pilot signals which have been inserted into the OFDM signal. The pilot signal has the previously defined amplitude and phase. The receiving device compares a received pilot signal with the pilot signal having the previously defined amplitude and phase to estimate the characteristic of the propagation path. The receiving device performs an equalizing process on a received signal on the basis of a result of path estimation performed.
The receiving device performs the path estimating process on the basis of a time-domain waveform signal obtained by performing Inverse Fast Fourier Transform (IFFT) processing on a plurality of pilot signals which have been inserted into the OFDM signal at fixed frequency intervals. A maximum analysis range which the path estimating process is allowed is limited by the frequency intervals of the pilot signals.
In addition, the receiving device is allowed to perform the path estimating process by calculating an autocorrelation of the received signal by using a correlation unit such as a matched filter or the like. The receiving device is allowed to increase the path estimable range and the accuracy in path estimation by combining a result of path estimation performed on the basis of the pilot signals with a result of path estimation performed on the basis of autocorrelation arithmetic processing. In addition, the receiving device is allowed to estimate the characteristic of the propagation path which is the frequency domain by performing Fourier transformation on the result of path estimation to transform the received signal to a frequency signal and then is allowed to perform propagation path compensation to compensate for the received signal on the basis of the estimated propagation path characteristic. Techniques relating to path estimating processes and propagation path compensation performed on the basis of the result of path estimation performed on the basis of the pilot signals or the result of path estimation performed on the basis of autocorrelation arithmetic processing are disclosed, for example, in Japanese Laid-open Patent Publication No. 2007-288450 and Japanese Laid-open Patent Publication No. 2008-72224. In addition, a technique relating to the mode 3 of ISDB-T which is the standard for digital broadcasting is disclosed, for example, in “TRANSMISSION SYSTEM FOR DIGITAL TERRESTRIAL TELEVISION BROADCASTING ARIB STD-B31”, Association of Radio Industries and Businesses, 2001.